1. Field of the Invention
The present invention relates generally to mechanical end face seals and more particularly relates to dual mechanical end face seals for use in sealing toxic or corrosive fluids.
2. Background Art
Mechanical end face seals have long been used to seal the space between a housing and a relatively rotating shaft which passes through the housing. Such seals usually include a primary ring which has a generally planar radial sealing face and is fixed to and mounted in the housing. The primary ring may be mounted on the housing by a secondary seal, such as an O-ring. In addition, the seal includes a mating ring which is mounted on the shaft for rotation therewith. Like the primary ring, the mating ring has a generally planar radial sealing surface. The mating ring radial surface is disposed in opposing relationship to the primary ring and the two radial surfaces are biased into sealing engagement. These types of seals are described in U.S. Pat. No. 4,212,475 issued to J. Sedy and assigned to the same assignee as the present invention.
Many configurations are known for utilizing either a single seal or a plurality of mechanical end face seals used together for specific seal applications. Single seal configurations are adequate for most sealing applications, including pumps, compressors, mixers and the like when utilized to seal fluids which are benign with respect to the environment. More recently, however, rising concern over pollution and toxic emissions has culminated in regulatory directives calling for "zero emissions" of toxic fluids into the environment. Thus, a need has arisen in the seal industry for seals which can provide a solution to the toxic fluid emission problem. Possible solutions which approached or met the zero emission standard have been proposed. These proposals have resulted in two broad categories of seal design, one type being known as "wet" double seals and the other type being magnetic drive pumps.
An example of the "wet" seals can be found in U.S. Pat. No. 4,290,611, issued to J. Sedy and also assigned with the present invention to a common assignee. That patent describes and illustrates a "double seal" arrangement (FIG. 1) which utilizes two mechanical and face seals oriented back to back along a drive shaft. The two seals define a chamber between them into which a lubricant buffer fluid is continuously circulated for cooling the seal rings. The buffer fluid, usually oil, is at a pressure generally 5-20 p.s.i. above the sealed process fluid pressure. The arrangement is described as being most desirable for sealing corrosive liquids because the metal parts of the seal are isolated from the process fluid by use of a non-corrosive buffer liquid.
The seal arrangements described in the '611 patent work well in certain applications, but cannot be used in applications where the sealed process fluid is a gas or where the sealed fluid is a liquid in which contamination by the buffer fluid cannot be tolerated. Generally, oil is used as a buffer fluid but many process fluids are reactive with the oil, or contamination by the oil in the process fluid is not desirable.
More recently, magnetic drive pumps have been developed which provide a "zero emission" capability, albeit at greater expense. For these types of applications, the shaft does not extend through the housing, but the shaft terminates at the housing wall thus eliminating the opening through which the shaft would extend. The impeller which pumps the fluid is encased in the housing chamber and is connected to a first set of magnets. The impeller is driven by a second set of magnets which are disposed externally of the chamber. Rotation of the externally disposed magnets by an external motor, in turn, rotates the magnets connected to the impeller inside the encasing housing chamber. Since the housing chamber is completely encased and does not include a shaft opening, no leakage of fluid can take place through the chamber wall under normal operating conditions.
The magnetic drive pumps are more complex and expensive than conventional mechanical end face seals. The magnets which drive the impeller are of special construction and special bearings are necessary to maintain the alignment of the magnets and the impeller shaft which provides the connection to the impeller. Moreover, magnetically driven pumps require a coolant fluid stream to remove waste heat generated by magnetic losses and by friction.
What is required by the industry is an inexpensive, easily constructed, seal which has a "zero emission" capability and which meets the regulations for toxic fluid emissions in an increasingly regulatory environment for general use.